As wireless communication systems for transmitting wideband digital signals such as video information or the like with high quality, wireless LAN and wireless video transmitting systems using microwaves or millimeter waves have been developed. Further, a sensor network configuration using a wireless system is being considered, in view of monitoring control in smaller factories or large-scale production facilities, anticrime monitoring for facilities, disaster-prevention monitoring for depopulated regions, and the like.
In a conventional wireless communication device, a transmitter side performs a modulating process or the like at an intermediate frequency of several tens of megahertz to several hundreds of megahertz or, in some cases, at an intermediate frequency in a several-GHz band at which signal processing can be easily performed. The transmitter side synthesizes the signal with a signal transmitted from a local oscillator to convert (up-convert) the frequency of the resultant signal into a transmission frequency in a 60-GHz band, for example. The transmitter side then emits the signal from an antenna through an electric supply line. A receiver side amplifies a wireless frequency signal transmitted from a receiving antenna, and synthesizes the amplified signal with a signal transmitted from the local oscillator to convert (down-convert) the frequency of the resultant signal into an intermediate frequency band. The receiver side then performs channel extraction and signal demodulation. This configuration is accompanied with a technical difficulty due to higher frequencies, and has problems such as deterioration of the efficiency of a local oscillator that is required to have a highly-stable and sufficiently large output, and deterioration of the efficiency of a wireless device such as a loss in a frequency converter. Furthermore, an increase in cost is a considerable obstruction to practical and wide use of wireless communication networks.
To address these disadvantages, attempts have been made to develop an active antenna in which an antenna and a wireless frequency circuit section are integrated to lower the costs, and there has been proposed a configuration of a radiating oscillator in which an amplifier element and an electromagnetic wave radiating structure are integrated with each other. The structure of this radiating oscillator is very simple, and is expected to be applied as a component of a low-cost wireless communication network.
Example configurations of wireless communication networks include Bluetooth which has already been put into practical use, and a series of IEEE 802.11 wireless standards known by the name of Wi-Fi. On the other hand, although the speed is a hundred times as low as the speed of the above-mentioned configurations, ZigBee, which is supposed to be able to form a network with lower power consumption and at low costs, is also being widely used as new means.
ZigBee is a technique compliant with the standard of IEEE 802.15.4 and is attractive as a short-distance, low-speed wireless link. In terms of the use of frequency, however, ZigBee is a wireless system of a 2.4 GHz band that is considerably congested in recent years, like Bluetooth or Wi-Fi.
On the other hand, when wideband digital information transmission by a 60-GHz-band millimeter wave which is high frequency is to be performed, if the phase noise and frequency stability of the local oscillator of the frequency converter are not sufficient, wideband digital signals cannot be transmitted with high quality due to frequency drift and phase noise. To counter this problem, a self-heterodyne wireless communication device technique and a wireless system configuration technique based thereon have been disclosed (see JP-A No. 2005-348332 (Patent Document 1) and JP-A No. 2003-198259 (Patent Document 2)).
In the self-heterodyne schemes disclosed in Patent Documents 1 and 2, although a local oscillator used in a transmitter is inexpensive and is of unstable frequency, a frequency shift or phase noise caused by the local oscillator is completely canceled at the time of signal detection, and it has been confirmed that signals are appropriately transmitted to a millimeter-wave-band wireless LAN or a wireless video transmission system which strictly requires phase noise and frequency stability.
Further, JP-A No. 2003-244016 (Patent Document 3) discloses frequency re-conversion required for configuring a wireless communication network system that is used for a relay to another system, for example. More specifically, a conversion to an intermediate frequency is once made and then re-conversion into a wireless transmission frequency is performed. At this point, however, deterioration in the quality of wideband digital signals is caused due to frequency drift and phase noise, if the phase noise or frequency stability of the local oscillator of the frequency converter is not sufficient. To counter this problem, a technique for configuring a system that wirelessly supplies a highly-stable reference signal has been disclosed.
JP Patent No. 3146260 (Patent Document 4), JP Patent No. 3355337 (Patent Document 5), and a document by R. A. Flynt, J. A. Navarro and K. Chang, ‘Low Cost and Compact Active Integrated Antenna Transceiver for System Application’, IEEE Trans. Microwave Theory Tech., Vol. 44, pp. 1642-1649, 1996 (Non-patent Document 1) disclose examples of radiating oscillators which respectively have a structure of a planar resonator using a transistor as a negative resistance amplifier. Non-patent Document 1 discloses a structure of a two-way transmission equipment in which radiating oscillators are arranged to face one another so that transmission/reception signals are polarized waves orthogonal to one another. According to Non-patent Document 1, this two-way transmission equipment is operated as a receiver by a mixer diode for frequency conversions.
Further, a document by C. M. Montiel, L. Fan and K. Chang, ‘A Self-Mixing Active Antenna for Communication and Vehicle Identification Applications’, 1996 IEEE MTT-S Digest, TU4C pp. 333-336, 1996 (Non-patent Document 2) discloses a technique by which a Gunn diode is used as an oscillation element. By this technique, a high-frequency voltage is superposed on a bias voltage of the Gunn diode to cause frequency modulation, and the modulated frequency is applied to another Gunn diode oscillator manufactured in the same manner. Accordingly, an injection synchronization phenomenon is caused. At the same time, the frequency modulation component is transmitted to another radiating oscillator, and, although two-way transmission and reception cannot be simultaneously performed through a mixer operation of the Gunn diode itself, a high-frequency signal component can be transferred.
As described above, the following techniques are being developed: a sensor network which avoids a large number of unsolved problems such as the problem of interference with an existing congested wireless system and the problems of interference and coexistence due to the high-density arrangement of wireless devices, and covers a large number of observation points in a wide region; a new wireless device technique for realizing a simple wireless communication network; and a wireless communication network configuration technique using the new wireless device technique.
However, by the conventional wireless device technique and the conventional wireless communication network configuration technique, the structures are complicated. As a result, the costs become higher, and there are problems in power consumption and signal quality. Those problems obstruct practical and wide use of wireless communication networks.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a wireless communication network system of a superhigh-frequency band to a millimeter-wave band that is suitable for a wireless communication system configuration and that has a very simple structure as the structure of a basic wireless communication device, attaining reduced costs and power consumption and high-quality signals.